1. Field of the Invention
This invention relates to fluoropolymer primer compositions useful for coating substrates, particularly copolymers based on ethylene and a comonomer selected from the group consisting of chlorotrifluoroethylene, tetrafluoroethylene and mixtures thereof.
2. DESCRIPTION OF THE PRIOR ART
Copolymers of about 40 to about 60 mol percent ethylene and correspondingly from about 60 to about 40 mol percent of chlorotrifluoroethylene, tetrafluoroethylene and mixtures thereof, are known in the art as disclosed, for example, in Mueller et al. U.S. Pat. No. 3,847,881, issued Nov. 12, 1974. As described therein, such copolymers may contain 3,3,3-trifluoro-2-trifluoromethyl propene to improve their stress-crack resistance. Also, as described in Carlson U.S. Pat. No. 3,624,250 issued Nov. 30, 1971, such copolymers may contain certain vinyl monomers in order to improve their stress-crack resistance. Such copolymers have been suggested as being useful for coating articles by powder coating techniques. However, it has been found that the adhesion between such copolymer coatings and the underlying substrate, particularly in the case of metal substrates, is too weak for certain applications, especially those wherein a severe environment is encountered. U.S. Pat. patent application Ser. No. 563,012, filed May 27, 1975, now U.S. Pat. No. 3,989,873 describes primer coatings for such copolymers which include about about 1 to 40 percent by weight of an oxide of cobalt, nickel, manganese, chromium, tin, molybdenum, tungsten or mixtures thereof. The disclosure of such application is expressly incorporated herein by reference. Although such primer coatings are effective for bonding such copolymers to metal and other substrates, it has been found that upon long term exposure to high temperature wet environments, the bond strength of coated articles substantially decreases. It would be desirable to provide a primer coating composition and coated articles based on such copolymers which have improved high temperature wet environment bond strengths.